1. Technical Field
The present invention relates to an image forming apparatus configured to form an image by ejecting ink from an inkjet head to a recording medium transferred along a transfer route.
2. Related Art
A line inkjet printing apparatus is one type of image forming apparatus. The line inkjet printing apparatus uses a long line inkjet head in which nozzles configured to eject ink are arranged over a length equal to or larger than the width of a printing area. The line inkjet printing apparatus forms an image by ejecting ink droplets from the nozzles of the inkjet head toward the recording medium below the inkjet head while moving and transferring the recording medium relative to the inkjet head in a transfer direction intersecting an arrangement direction of the nozzles without moving the inkjet head.
In such a line inkjet printing apparatus, ink chambers communicating with the nozzles are provided inside the inkjet head and the capacities of the ink chambers are changed (increased and decreased) by using drive signals to eject the ink droplets from the nozzles. Accordingly, when the number of the nozzles is increased to improve the resolution in a main scanning direction which is the nozzle arrangement direction, two nozzle arrays are provided in one inkjet head at positions offset from each other in the transfer direction in order to secure an arrangement space for the ink chambers in the inkjet head in which a dimension in the main scanning direction is limited.
When two nozzle arrays offset from each other in the transfer direction are provided in the inkjet head, the nozzle arrays are arranged to be offset from each other by a half pitch in the main scanning direction. The ink chambers are thus arranged in the inkjet head in a zigzag pattern and the resolution can be made higher than that in a case where the ink chambers are arranged in a straight line in the main scanning direction.
When the recording medium is transferred to a position directly below the head, an air flow (hereafter, referred to as transfer air flow) from an upstream side to a downstream side in the transfer direction is generated. Accordingly, in a non-contact printing method of ejecting the ink droplets from the nozzles to the recording medium, the ink droplets are affected by the transfer air flow and are swept downstream in the transfer direction of the recording medium. As a result, so-called landing deviation in which the ink droplets deviate from desired trajectories and applied onto the recording medium occurs. This is a factor causing deterioration in image quality.
To counter such a problem, in Japanese Patent Application Publication No. 2010-173178, when ink droplets are ejected while an inkjet head having multiple nozzles and a recording medium are moved relative to each other in a direction intersecting an arrangement direction of the nozzles, the ejection is controlled in such a way that the ejection is controlled in such a way that the droplet with a smaller amount is ejected at a faster speed. This control can suppress the landing deviation of ink droplets due to the transfer air flow.